Solid-State Nanopores: Methods of Fabrication and Integration, and Feasibility Issues in DNA Sequencing - Nanopores: Sensing and Fundamental Biological Interactions - page 185

Besides the abrupt increase in current, it was found that one can measure the
simultaneous cyclic voltammogram at a certain rate to check the existence of a pore.
The current-voltage ( I - V ) curves show different characteristics before and after the
opening of a pore. In the absence of a pore, the I - V curve is non-ohmic over a wide
range of bias voltage, in agreement with prior studies of electrochemical KOH
etching of silicon. On the contrary, in the presence of a pore, the I - V curve shows
ohmic characteristic.
It was found that the post-etch pore ionic resistance gives a good measure of the
pore size, correlating strongly with the stoppage current, the electric current level at
which the etching process is terminated. To properly determine the ionic resistance
(or conductance) of the pore, the I - V measurements are performed with both sides
of the silicon chip filled with 1 M KCl solutions and using Ag/AgCl wires as
electrodes. Figure 8.6 shows TEM pictures of a sample obtained at a stoppage
current of 248 nA (at 800 mV) and after 18-min oxidation. It was found that the
pores can be shrunk with additional oxidation time. In fact a similar surface-tension
driven shrinking process was invoked [ 9 ].
Park et al. [ 9 ] have demonstrated that this simple benchtop method can be used
for fabricating nanopores in silicon chips in the range of 3-50 nm. This technique
only relies on standard silicon processing equipment, with no requirement of
specialized ion-beam and electron-beam facilities, making solid-state nanopore
technology affordable for a wide range of users.

Fig. 8.6 TEM images showing the morphology of (a) an as-etched and (b) an oxidized pore.
The as-etched silicon sample has a resistance of 6.7 M (1 M KCl, Ag/AgCl electrodes).

(a) Rectangular-shaped pore is attained with dimensions of 15 nm

O

38 nm. (b) After oxidation
at 1,100 C for 1 min, the pore becomes smooth and round (10 nm long axis and 5 nm short axis)
[ 9 ]. Used with permission. Copyright Small 2007